When A.C. Signals are coupled to an ADC, the average or D.C. component must be established at a predetermined voltage level in order to efficiently utilize the dynamic range of the ADC. In addition, it is frequently desired to multiplex a plurality of signals to the input of a common ADC, and to insure that the D.C. levels of the plurality of signals are clamped to the same level. For example, in television receivers which digitally process modulated color component signals separately, it is necessary to insure that the average value (e.g., zero) of the component signals is accurately established in order to avoid introducing color errors in the reproduced images.
A typical ADC input clamp circuit is described in U.S. Pat. No. 4,859,871, wherein a comparator is coupled at the output of an ADC to provide clamp control signals to a D.C. clamping circuit at the input of the ADC. During intervals when the signal amplitude should be at a known or predetermined level, the signal is compared against a fixed reference to provide a measure of the difference between the desired D.C. input signal level and the actual D.C. input signal level. The difference value is arranged to cause the clamping circuit to adjust the actual D.C. level in the direction to attain the desired level. At the end of the comparison interval the D.C. adjusting circuitry is effectively disconnected from the input signal path. The input D.C. level is held until the next comparison interval by virtue of being stored on a capacitor in the input signal circuit path.
The comparison circuitry of known ADC clamp circuits tends to be relatively complicated. According to the present invention the comparison circuitry of ADC input clamping circuitry may be eliminated.